Sixteen experiments investigate the development, in human infants, of knowledge of the immediately perceivable world. The experiments focus primarily on infants' knowledge of physical objects, and they investigate whether infants are sensitive to 5 principles governing object behavior: continuity (objects move on connected, unobstructed paths), cohesion (objects move as connected, bounded units), contact (objects act upon each other only on contact), gravity (objects move downward in the absence of support), and inertia (objects move smoothly in the absence of obstacles). Infants' knowledge in investigated by means of three methods: (1) a reaching method, focusing on infants' ability to reach predictively for a moving object by extrapolating its motion, (2) a standard preferential looking method, focusing on infants' tendency to look longer at events in which a visible object moves anomalously and (3) and invisible displacement preferential looking method, focusing on infants' tendency to look longer at the outcomes of events in which an object moves from view and then reappears at an impossible location. If infants are sensitive to a principle governing object motion, then they are expected to reach for a moving object by extrapolating its motion in accord with that principle, and they are expected to look longer at visible or partly hidden events in which an object's motion violates this principle. The long-term objectives of this proposal are (1) to elucidate core human conceptions of the world through studies of their origins in infancy, (2) to assess the accessibility, generality, and strength of infants' understanding of their surrounding, and (3) to elucidate the mechanisms by which knowledge is acquired, through study of the acquisition process. An understanding of core physical knowledge and its development might shed light on the structure and acquisition of knowledge more generally, and it should aid efforts to facilitate knowledge acquisition in formal science instruction. In the future, such understanding may contribute to the detection and treatment of children with early developing cognitive impairments.